Of all the fish in the world, only a few have the capacity to maintain warmth in specific parts of their bodies. But as new research reveals, the deepwater opah has the unprecedented ability to circulate heated blood throughout its entire body, making it the only known fully warm-blooded fish.

Top image: Elevated temperature in the eye and brain of the opah allow for enhanced vision. (credit: NOAA Fisheries, Southwest Fisheries Science Center)

Warm-blooded animals, known to scientists as endotherms, can conserve metabolic heat and maintain it above the ambient temperature of their environment. For mammals and birds, these characteristics confer a number of benefits, including quicker reaction times, increased muscle power output, and improved aerobic performance.

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Small wonder, then, that some predatory fish have evolved something resembling warm-bloodedness; both tuna and lamnid sharks, for example, boost their swimming performance by heating their muscles. But their ability to do so is limited; the internal organs of these quasi-endothermic fish cool off quickly when they slow down, or descend into colder waters. By strict definition, therefore, these fish are not true endotherms; they’re what’s referred to as “regional endotherms.” Among all fish, a scant 0.1% have evolved this capacity.

Enter the opah, a deep sea critter commonly referred to as the moonfish that scientists now report is a bona fide, whole-body endotherm.

Generate and Conserve

The opah (Lampris guttatus) typically resides at depths of 150- to 1,300-feet. In these chilly waters, this round, tire-sized fish swims by vigorously flapping its prominent pectoral fins. In the latest issue of Science, researchers led by fisheries biologist Nicholas Wegner, of NOAA Fisheries’ Southwest Fisheries Science Center, demonstrate that this constant, bird-like flapping motion is the source of the opah’s internal heat. But generating warmth is only half the battle. To qualify as endotherms, these fish must also conserve their hard-earned heat and circulate it through other organs like the heart and brain. In their report, Wegner and his colleagues report that opahs accomplish this by virtue of their unique gill structure.

While sampling opah gill tissue, the NOAA researchers noticed a design that is familar to biologists and engineers alike: counter-current heat exchange. Within their gills, these fish have a complex array of tightly packed arteries and veins—a feature scientists call the rete mirabile (latin for “wonderful net”)—that prevents a loss of heat across the body surface while also preventing cold blood from entering the core of the body. This allows the opah to preserve a body temperature warmer than its surrounding waters. (Image: N. C. Wegner et al./Science)

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(University of Alberta)

“The discovery that the opah can warm up its entire body changes the way we think about endothermy,” Wegner told io9. “If you had asked me a few years ago if a fish could warm up its entire body, I would have said it was extremely unlikely. This just goes to show how little we still know about many organisms and how much there is still left to discover.”

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Further real-world analysis on the fish themselves revealed that their entire body core and brain regions were all significantly warmer than their environment.

Body temperature in the opah. (N.C. Wegner et al./Science)

For example, pectoral muscle temperature among free-swimming opah ranged between 1.2 to 4.8 degrees C above ambient temperature in waters ranging from around 7.8 to 10.8 degrees C and at depths of 150 to 1,000 feet.

A Competitive Advantage

As noted, being warm-blooded has its advantages. For the opah, that translates to speed. To the surprise of the researchers, this fish is not the slow and and lumbering fish it’s typically thought to be. In reality, it’s an active predator despite its strange body shape.

The NOAA scientists provided an excellent summary of the potential benefits in their study:

With a warm body core and heart, and even warmer cranial region, opah have the capacity for enhanced physiological function in their deep, cold habitat. The elevated body temperature of opah should increase muscle power output and capacity for sustained performance, enhance temporal resolution and neural conductance for the eye and brain, increase the rates of food digestion and assimilation in the digestive tract, and reduce the impact of cold ambient temperatures and temperature changes on cardiac and other organ performance. Supporting its endothermic ability and increased aerobic performance, the opah has a relatively large heart and gill surface area, high hematocrit level, and an unusually large aerobic muscle mass, all of which are similar to characteristics of high-performance predators such as tunas and lamnid sharks, and in stark contrast to those of other fishes from its order...which tend to be slow-moving ambush predators.

Given all these advantages, it’s reasonable to wonder why more fish haven’t evolved this remarkable capacity.

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“I think the reason more fish don’t have this capability is because of the difficulty in maintaining a warm body in water,” explained Wegner to io9. “Water has a very high heat capacity (meaning it can readily absorb a lot of heat). Fish are at a distinct disadvantage for retaining internal heat because they breathe water and thus blood comes in close contact with the water at the gills (where heat can easily be lost).”

As a result, Wegner says it takes a number of very specific adaptations, such as the counter-current heat exchangers, to minimize heat loss to the environment.

Given the discovery of several distinct opah species, the NOAA scientists say “the stage [is set] for future comparative studies to further explore this key evolutionary innovation.”